determining infiltration rates

May 16, 2017

Presented by

Andrew Austreng, LHG

Determining Infiltration Rates: Approaches, Challenges & Lessons Learned

Overview of Site Characterization & Testing

Pilot Infiltration Test (PIT) Specifics

Approaches and Challenges

Lessons Learned &Tools Available

Presentation Outline

Infiltration Testing: Site Characterization

Suitability Criteria• Max Infiltration Rates with Treatment

• Western: 3 in/hr (design)• Eastern: 2.4 in/hr

• Minimum Rate without Treatment• Western: 48-hr drain• Eastern: 72-hr drain

• 5’ separation between GW and/or low k (3’ possible)

• Setback from 15% slope

Subsurface Investigation (Report)

Subsurface Investigation: Approaches

Western WA• Assume 0.5 in/hr for investigation area• 5x pond depth (at least 10 ft)

• Characterize GW (<50 ft) - monitor 1 wet season

Eastern WA• 3 excavations within pond• 5 ft below bottom (at least 10 ft if no info available)

Key to determining mounding potential…..

Subsurface Investigation: Existing Info

Geologic Maps/Studies• Consider Scale• Evaluate Source of Info

Eastern WA: Determining Infiltration Rates

Four Recommended Tests1. Borehole (deep or high k)

2. Drywell (confirmatory)

3. Ring Infiltrometer (surficial or high k)

4. Test PitSingle-Ring Inflitrometer

100 ft 2 PIT (large-scale)

Western WA: Infiltration Testing

Three Recommended Tests1. Large-scale PIT (100 ft2)2. Small-scale PIT (12-36 ft2) < 1 acre High rates Permeable pavement, etc.

3. Grain Size When not compacted (recessional outwash)

PIT: Eastern & Western Procedures

Flow Stable at 5% variation

Western more conservative (mounding)

Western EasternDepth of Test Pit Bottom 2 to 5 ft Below

Saturation Time > 5hrs < 2hrsStable Flow ≥1 hr 0.5 hrsFalling Head Until Dry 30 min

Pit Area 12 - 100 ft2 8 ft2Testing Season 12/1 – 4/1 --

PIT Method: Qualitative Mounding (Western)

Boreholes

Over-excavate

Eastern WA: PIT Calculations

Reclamation ProcedureLow Water Table: High Water Table:

• Determines permeability coefficient (not gradient)

Western WA: PIT Calculations

Calculations• Ki = Q/A

𝟓𝟓𝟓𝟓 𝒈𝒈𝒈𝒈𝒈𝒈 = 400𝑓𝑓𝑓𝑓3

ℎ𝑟𝑟 ;400 𝑓𝑓𝑓𝑓3ℎ𝑟𝑟−1

𝟏𝟏𝟓𝟓𝟓𝟓 𝒇𝒇𝒇𝒇𝟐𝟐 = 𝟒𝟒 𝒇𝒇𝒇𝒇/𝒉𝒉𝒉𝒉

Correction Factors

Issue FactorHeterogeneity 0.33 - 1Large PIT 0.75Small PIT 0.5Other (small scale) 0.4Grain Size 0.4Long-term fouling 0.9

= 0.12 to 0.68

Western WA: Calculations (Detailed Design)

Described in Western for:• Drainage > 10 acres• Low k within 15 feet (10 feet)

6. Calculate Conservative Gradient (mounding)

7. Correct for Aspect ratio (pit bottom)

8. Size for 48-hr drawdown (6 ft max head)

9. Run MODRET (or other analytical)

Western WA: Grain Size Calculations

Calculations:• Massmann (2003)• Harmonic mean, or• Lowest k if w/in 5 feet

Need to Consider:• Hydraulic gradient• Soil structure (e.g., varves, laminae)

• Compaction during construction

Western WA: Grain Size Calculations

Confirm with Other Methods….

Test PitHydraulic Conductivity (ft/hr)

PIT ResultsMassmann USBR Pavchich

PIT 1 5.1 0.9 1 0.9 to 1.2

PIT 2 2.9 0.4 0.4 0.4 to 0.5

…..WDOT

PIT Method: Data Collection Tips

Dimensions are Critical!!

• Ki = 𝑄𝑄𝑨𝑨

Consider Shoring

PIT Method: Data Collection Tips

Flow Metering• Adjusting Valves (5% threshold)

• Rate Will Vary with Head

Butterfly Ball Gate

MJ Series

PIT Method: Data Collection Tips

Automation• Pressure Transducer• Pulse Meter + datalogger

PIT Method: Challenges

“Constant Head”

Infiltration Feasibility Assessment for Seattle-Tacoma International Airport

2017 Washington Municipal Stormwater Conference

Tom Atkins, PE, LG

Presentation Outline STIA Overview

Airport layout and operations Stormwater management Geology/hydrogeology Need for infiltration assessment

Approach Results

Seattle-Tacoma International AirportProject Location

Located in City of SeaTac 12 miles south of downtown Seattle

Largest airport in the Pacific Northwest 1600 acres with 3 streams discharging to

Puget Sound 45 million passengers in 2016 Largest generator of vehicle trips in the

state and a 13,000-car parking garage* Substantial future development plans

STIA Overview

Study Area

Stormwater Management 2 Collection and Conveyance Systems

Industrial Wastewater System (IWS) Aircraft/vehicle maintenance

Storm Drainage System (SDS) Runways/expressways, terminal, service roads

Individual NPDES Permit IWS Non-construction stormwater Construction stormwater

Low Impact Development Port’s LID Guidelines being developed Stormwater Management Manual for

Aviation Division Property being updated Broad-scale infiltration assessment

needed to provide guidance for identifying future stormwater infiltration opportunities

Assessment primarily focused on hydrogeologic considerations

Determine important factors Identify and obtain existing available

information Create GIS layers of each factor Determine infiltration feasibility of each

unique combination of factors Create summary shallow and deep

infiltration feasibility maps

Infiltration Assessment Approach

Substantial amount of existing information LiDAR elevation and Port topographic survey

data Surficial geologic maps Subsurface geologic and hydrogeologic

information from the 2008 Groundwater Study Sensitive and critical areas Other notable information

Information Sources

Topographic Layer

Created using LiDAR that was adjusted to post-Third Runway conditions using Port survey data

Ground elevations range from 60 ft(southwest area) to over 500 ft above mean sea level (central area)

Ground Surface Elevation

Surficial Geologic Units

Based on regionally significant surficial geologic units modeled in 2008 Groundwater Study

Also identified Airport fill and regraded areas

Geology/Hydrogeology

Puget Sound lowland glaciation resulted in glacially sculpted uplands

Post-glacial erosion has locally incised the uplands and created steep-sided ravines

2008 groundwater study identified 12 regional hydrostratigraphic units

Vashon glacial till (Qvt) covers much of STIA’s high plateau area

Advance outwash (Qva) stratigraphically below the glacial till

Surface Geology

Cross-Section B-B’

Sensitive, Critical and Other Notable Areas Layer

Streams, ponds, lakes and wetlands with 100-ft buffers included

FAA regulated areas Runway Safety Areas Object-free Areas Protection Zones

Areas of potential soil contamination Municipal water supply wells

Sensitive, Critical and Other Areas

Infiltration Feasibility Factors Surface geology/gross unit hydraulic

conductivity Surface slope gradient Proximity to steep slopes and landslide

hazard areas Thickness of permeable unsaturated

zone Depth to permeable receptor unit

Surface Geology/Permeability Geologic units categorized into broad permeability

categories

High permeability Vashon Recessional Outwash Qvr

Moderate Permeability Alluvium Qal

Vashon Advance Outwash Qva

Older coarse-grained deposits Qpf

Low Permeability Vashon Glacial Till Qvt and other fine-grained deposits

Surface Permeability

Surface Slope Gradient

Surface slope was divided into three categories Low Gradient (<8%)

Moderate Gradient (8 – 20%)

High Gradient (>20%)

Surface Slope

Steep Slope Hazard Areas

Two landslide hazard categories High to Moderate Landslide Hazard

Slopes >40% plus 100-ft buffer or within 500 ft of a mapped landslide/steep hazard

Low Landslide Hazard All other areas except embankment fill

Infiltration to Third Runway embankment fill is prohibited Fill area estimated by comparing pre- and post-

construction topography with a 500 ft buffer east of upper fill

Steep Slope Hazard Areas

Depth to Permeable Unsaturated Zone

Three depth categoriesShallow depth to infiltration receptor

(<20 ft)

Moderate depth to infiltration receptor (20-50 ft)

Deep infiltration receptor (>50 ft)

Depth to Subsurface Permeable Unsaturated Zone

Thickness of Permeable Unsaturated Zones

Three thickness categoriesHigh thickness (>30 ft)

Moderate thickness (10 – 30 ft)

Low thickness (<10 ft)

Thickness of Subsurface Permeable Unsaturated Zone

Hydrogeomorphic Units

Each unique combination of infiltration feasibility factors defines a hydrogeomorphic unit

Shallow and deep infiltration hydrogeomorphic units determined

Criteria for Shallow Infiltration Hydrogeomorphic Unit Categories

Feasibility Permeability Surface SlopeUnsaturated Zone

ThicknessSlope

Hazard

GoodGenerally the most favorable rating with up to one

moderate ratingLow

ModerateGenerally the most favorable rating with up to two

moderate ratingsLow

PoorGenerally one or more least favorable rating. Low

Any rating High to moderate

Criteria for Deep Infiltration Hydrogeomorphic Unit Categories

FeasibilityUnsaturated Zone

ThicknessDepth to Permeable Unsaturated Zone Slope Hazard

Good >10 feet < 50 feet LowModerate >10 feet Any depth Low

Poor <10 feet Any depth Low

Any rating High to moderate

Shallow Infiltration Feasibility

Deep Infiltration Feasibility

Summary of Results Over half of the study area is not expected to

be suitable for shallow infiltration due to the presence of low permeability glacial till soils or other factors

Deep infiltration appears feasible in significant portions of the study area, including many areas that were identified as being unsuitable for shallow infiltration

Areas with relatively thin till cap underlain by outwash soils supports the feasibility of dug or drilled drains

Recent Infiltration Testing Results

Infiltration testing – conducted in-conjunction with a planned environmental remediation effort at Lake Reba –demonstrated good infiltration

Result consistent with shallow infiltration feasibility mapping

Questions?

Aspec t Consu l t ing , LLCStormwater ~ Hydrogeo logy ~ Water Resources ~ Data Serv ices

Geotechn ica l Eng ineer ing ~ Env i ronmenta l Remedia t ion

B a i n b r i d g e I s l a n d – B e l l i n g h a m – S e a t t l e – W e n a t c h e e – Y a k i m a – P o r t l a n d

Tom Atkins, PE, LGtatkins@aspectconsulting.com

206.418.8207

Andrew Austreng, LHGaaustreng@aspectconsulting.com

701.740.7915